The invention relates to a circuit assembly for generating pulses sustaining or xe2x80x9cpluckingxe2x80x9d RF oscillation in a resonant circuit of a batteryless transponder in which the supply voltage needed for its operation is obtained from a RF carrier oscillation pulse defined in time exciting the resonant circuit into oscillation and used to charge a storage element whose charging voltage forms the supply voltage, in which a plucking pulse is generated every time the amplitude of the RF oscillations drops below a defined threshold value and its momentary value is in a defined relationship to a reference voltage changing in time as the charging voltage of a capacitor and wherein a switch is provided which can be switched on for the duration of the plucking pulse for connecting the storage capacitor to the resonant circuit.
Described in EP 0 301 127 B1 is a transponder which works without a battery and whose supply voltage is obtained from a RF carrier oscillation pulse received by the transponder. Provided in the transponder is a resonant circuit whose coil simultaneously serves as an antenna for receiving the RF carrier oscillation pulse. Rectifying the received RF carrier oscillation pulse generates a DC voltage with which a storage capacitor is charged, whose charging voltage, on conclusion of the RF carrier oscillation pulse, is employed as the supply voltage for the transponder. The simplest typical application of one such transponder is its use as an identification element. In this arrangement it is configured so that at the end of the received RF carrier oscillation pulse it is in turn able to transmit an RF signal which is modulated with an ID code stored in the transponder. An interrogator is thus able to read this ID code and accordingly also able to establish the identity of items to which the transponder is secured.
Transmitting the RF signal by the transponder requires the resonant circuit contained therein to continue to oscillate after termination of the RF carrier oscillation pulse. These oscillations of the resonant circuit are used to generate clock signals in the transponder needed to modulate the RF oscillation so that this signal can then be transmitted in modulated form by the coil of the resonant circuit. Depending on the Q of the resonant circuit and further unavoidable damping factors, the oscillations excited in the resonant circuit by the RF carrier oscillation pulse decay more or less quickly aperiodically. This is why it needs to be assured in the known transponder that energy is supplied from the storage capacitor to the resonant circuit in fixed time intervals which keep the oscillation going as long as energy is still stored.
Described in DE-A-39 14 888 is a circuit assembly with the aid of which the energy supply to the resonant circuit of the transponder can be optimized. This circuit ensures that the energy supply is made to the resonant circuit in the sense of a positive feedback at each optimum point in time during a half-cycle of the RF oscillations. In a transponder of this type available commercially from Texas Instruments as TMS 3789, TMS 3791 or TMS 3792 this principle of plucking the RF oscillations in the resonant circuit of the transponder is employed in which the energy supply is made each time after a fixed number of RF cycles. It is possible to vary the setting of this number of RF cycles for the various transponders for adapting them to the various damping conditions anticipated in practical operation. This is, however, not optimal for changing damping conditions, since it may happen that when the resonant circuit is strongly damped, the energy supply for plucking the RF oscillations occurs too late to be effective or, when the damping is less, the energy supply occurs too often and thus fails to make optimum use of the energy available in the storage capacitor, it instead being wasted.
The invention is based on the object of configuring a circuit assembly of the aforementioned kind so that optimum use of the energy available in the storage element is achieved.
In accordance with the invention this object is achieved in that a closed control loop is provided which varies the slope of the reference voltage curve between two plucking pulses in the direction of maintaining the predefined relationship between the momentary value of the RF oscillations and the reference voltage.
In making use of the circuit assembly in accordance with the invention, energy is supplied to the resonant circuit as controlled by the plucking pulse not only at each correct point in time but also in adapting to each Q of the resonant circuit as dictated by the properties of the components of the resonant circuit and by the external damping effects. Energy is now supplied only when actually needed to sustain the RF oscillations, i.e. thus avoiding energy being supplied to the resonant circuit when already capable of continuing to oscillate for a certain period without this energy supply.
Advantageous aspects of the invention read from the sub-claims.
A transponder provided with the circuit assembly in accordance with the invention can be put to use in a wide variety of applications without its parameter influencing the Q of its resonant circuit needing to be changed, i.e. the circuit assembly in accordance with the invention automatically adapts to the change in the Q of the circuit which greatly expands the range of possible applications of the transponder.